Process for purifying dinitrogen monoxide

ABSTRACT

The present invention relates to a process for purifying a gas mixture comprising dinitrogen monoxide, at least comprising the at least partial condensation of a gas mixture G-I comprising dinitrogen monoxide to obtain a liquid composition C-1 comprising dinitrogen monoxide, and the contacting of the composition C-1 with a gas mixture M-1 to obtain a composition C-2 and a gas mixture M-2.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. §371)of PCT/EP2009/052992, filed Mar. 13, 2009, which claims benefit ofEuropean application 08153953.8, filed Apr. 2, 2008.

BACKGROUND OF THE INVENTION

The present invention relates to a process for purifying a gas mixturecomprising dinitrogen monoxide, at least comprising the at least partialcondensation of a gas mixture G-I comprising dinitrogen monoxide toobtain a liquid composition C-1 comprising dinitrogen monoxide, and thecontacting of the composition C-1 with a gas mixture M-1 to obtain acomposition C-2 and a gas mixture M-2.

The prior art discloses various preparation processes and purificationprocesses for dinitrogen monoxide. It is likewise known that dinitrogenmonoxide can be used, for example, as an oxidizing agent for olefins.

For instance, WO 98/25698 discloses a process for preparing dinitrogenmonoxide by catalytic partial oxidation of NH₃ with oxygen. According toWO 98/25698, a catalyst composed of manganese oxide, bismuth oxide andaluminum oxide is used, which leads to dinitrogen monoxide with highselectivity. A similar catalyst system is also described in detail in ascientific study (Noskov et al., Chem. Eng. J. 91 (2003) 235-242). U.S.Pat. No. 5,849,257 likewise discloses a process for preparing dinitrogenmonoxide by oxidation of ammonia. The oxidation takes place in thepresence of a copper-manganese oxide catalyst.

In the process disclosed in WO 00/01654, dinitrogen monoxide is preparedby reducing a gas stream comprising NO_(x) and ammonia.

The oxidation of an olefinic compound to an aldehyde or a ketone bymeans of dinitrogen monoxide is described, for example, in GB 649,680 orthe equivalent U.S. Pat. No. 2,636,898. Both documents quite generallydisclose that the oxidation can in principle be effected in the presenceof a suitable oxidation catalyst.

The more recent scientific articles of G. I. Panov et al.,“Non-Catalytic Liquid Phase Oxidation of Alkenes with Nitrous Oxide. 1.Oxidation of Cyclohexene to Cyclohexanone”, React. Kinet. Catal. Lett.Vol. 76, No. 2 (2002) p. 401-405, and K. A. Dubkov et al.,“Non-Catalytic Liquid Phase Oxidation of Alkenes with Nitrous Oxide. 2.Oxidation of Cyclopentene to Cyclopentanone”, React. Kinet. Catal. Lett.Vol. 77, No. 1 (2002) p. 197-205 likewise describe oxidations ofolefinic compounds with dinitrogen monoxide. A scientific article“Liquid Phase Oxidation of Alkenes with Nitrous Oxide to CarbonylCompounds” by E. V. Starokon et al. in Adv. Synth. Catal. 2004, 346,268-274 also includes a mechanistic study of the oxidation of alkeneswith dinitrogen monoxide in the liquid phase.

The synthesis of carbonyl compounds from alkenes with dinitrogenmonoxide is also described in various international patent applications.For instance, WO 03/078370 discloses a process for preparing carbonylcompounds from aliphatic alkenes with dinitrogen monoxide. The reactionis carried out at temperatures in the range from 20 to 350° C. andpressures of from 0.01 to 100 atm. WO 03/078374 discloses acorresponding process for preparing cyclohexanone. According to WO03/078372, cyclic ketones having from 4 to 5 carbon atoms are prepared.According to WO 03/078375, cyclic ketones are prepared under theseprocess conditions from cyclic alkenes having from 7 to 20 carbon atoms.WO 03/078371 discloses a process for preparing substituted ketones fromsubstituted alkenes. WO 04/000777 discloses a process for reacting di-and polyalkenes with dinitrogen monoxide to give the correspondingcarbonyl compounds. The purification of dinitrogen monoxide is notmentioned in these documents.

It is likewise known that offgas streams comprising dinitrogen monoxidecan be used for further reactions. Dinitrogen monoxide is obtained as anundesired by-product in various chemical processes, especially inoxidations with nitric acid and there very particularly in the oxidationof cyclohexanone and/or cyclohexanol to adipic acid. Other examples ofprocesses in which dinitrogen monoxide is obtained as an undesiredby-product are the oxidation of cyclododecanone and/or cyclododecanolwith nitric acid to give dodecanedicarboxylic acid, the oxidation ofacetaldehyde with nitric acid to glyoxal and the partial oxidation ofNH₃ to NO.

For instance, WO 2005/030690, WO 2005/030689 and WO 2004/096745 discloseprocesses for oxidizing olefins with dinitrogen monoxide, specificallythe oxidation of cyclododecatriene, of cyclododecene and ofcyclopentene. All three applications disclose that, in addition to otherdinitrogen monoxide sources, it is also possible to use offgas streamswhich can be purified, for example, by distillative methods before theyare used as oxidizing agents.

Both in the preparation of dinitrogen monoxide and in the use of offgasstreams, N₂O is obtained initially as a dilute gaseous mixture withother components. These components can be divided into those which havea disruptive effect for specific applications and those which behaveinertly. For use as an oxidizing agent, gases having a disruptive effectinclude NO_(x) or, for example, oxygen (O₂). The term “NO_(x)”, asunderstood in the context of the present invention, refers to allcompounds N_(a)O_(b) where a is 1 or 2 and b is a number from 1 to 6,except N₂O. Instead of the term “NO_(x)”, the term “nitrogen oxides” isalso used in the context of the present invention. Disruptive secondarycomponents also include NH₃ and organic acids.

For specific applications, it is necessary to purify the dinitrogenmonoxide used before the reaction. For example, for the use ofdinitrogen monoxide as an oxidizing agent, it is necessary to removedisruptive secondary components such as oxygen or nitrogen oxidesNO_(x).

Processes for removing NO_(x) are known in principle from the prior art.A review is given, for example, by M. Thiemann et. al in Ullmann'sEncyclopedia, 6th Edition, 2000, Electronic Edition, Chapter “NitricAcid, Nitrous Acid, and Nitrogen Oxides”, Section 1.4.2.3.

The application WO 00/73202 describes a method as to how NO_(x) and O₂can be removed from an N₂O-containing gas stream. The NO_(x) is removedby catalytic reduction with NH₃ and oxygen by catalytic reduction withhydrogen or other reducing agents. However, this method has thedisadvantage that the product is contaminated with NH₃. A high depletionof oxygen is possible only when a loss of N₂O is accepted (of, forexample, from 3 to 5% of the amount originally present).

For specific applications, it may be necessary also to remove the inertcompounds, since they can slow the desired reaction with N₂O bydilution. The term “inert gas”, as used in the context of the presentinvention, refers to a gas which behaves inertly with regard to thereaction of N₂O with an olefin, i.e. reacts under the conditions of thereaction of olefins with N₂O neither with the olefins nor with N₂O.Inert gases include, for example, nitrogen, carbon dioxide, argon,methane, ethane and propane. However, the inert gases can lower thespace-time yield, so that a depletion can likewise be advantageous.However, it may likewise be advantageous to obtain a gas mixture whichstill comprises inert gases, such as carbon dioxide, and then can beused directly in a further reaction.

DE 27 32 267 A1 discloses, for example, a process for purifyingdinitrogen monoxide, wherein nitrogen oxide, nitrogen dioxide, carbondioxide and water are initially removed and the gas mixture issubsequently liquefied by compression to from 40 to 300 bar and coolingto from 0 to −88° C. From this liquefied gas mixture, dinitrogenmonoxide is then removed. Although this method achieves a purificationand concentration of the N₂O, it is economically unattractive owing tothe required high pressure (60 bar), the low temperatures (−85° C.) andthe associated high capital costs.

U.S. Pat. No. 4,177,645 discloses a process for removing dinitrogenmonoxide from offgas streams which likewise comprises a prepurificationand a low temperature distillation. The application EP 1 076 217 A1likewise describes a method for removing low-boiling impurities from N₂Oby low temperature distillation.

U.S. Pat. Nos. 6,505,482, 6,370,911 and 6,387,161 also discloseprocesses for purifying dinitrogen monoxide, in which a low temperaturedistillation is in each case carried out in a special plant.

However, as a result of the high pressures and low temperatures, a lowtemperature distillation entails high apparatus demands, which make thepurification of the dinitrogen monoxide with such a process inconvenientand costly. Particularly troublesome in this context is the fact thatthe melting point of N₂O at standard pressure is only 3 K below theboiling point. It is therefore necessary to employ high pressures.

DE 20 40 219 discloses a preparation process for dinitrogen monoxide,wherein the dinitrogen monoxide obtained is concentrated and purifiedafter the synthesis. According to DE 20 40 219, dinitrogen monoxide isprepared initially by oxidizing ammonia. The dinitrogen monoxideprepared is purified by separating the oxidized gases and concentratingby absorption under high pressure, which is followed by a desorptionunder reduced pressure. Secondary components are removed, for example,by treatment with an alkali solution in a wash tower. According to DE 2040 219, water is used as the solvent for the absorption of the gasmixture.

It is possible with the process disclosed in DE 20 40 219 to separatethe different nitrogen oxides, but the process entails the use of largeamounts of solvent and/or high pressures for the absorption. At the sametime, a further wash tower is needed for the process disclosed in DE 2040 219 to remove further disruptive components.

WO 2006/032502 discloses a process for purifying a gas mixturecomprising dinitrogen monoxide, which comprises at least one absorptionof the gas mixture in an organic solvent and subsequent desorption ofthe gas mixture from the laden organic solvent, and also the adjustmentof the content of nitrogen oxides NO_(x) in the gas mixture to at most0.5% by volume based on the total volume of the gas mixture. WO2006/032502 also discloses that the process may comprise a plurality ofabsorption and desorption steps. WO 2006/032502 discloses only organicsolvents as the absorption medium.

DE 10 2005 055588.5 relates to a process for purifying a gas mixture G-0comprising dinitrogen monoxide, at least comprising the absorption ofthe gas mixture G-0 in an organic solvent, subsequent desorption of agas mixture G-1 from the laden organic solvent, absorption of the gasmixture G-1 in water and subsequent desorption of a gas mixture G-2 fromthe laden water, and to the use of a purified gas mixture comprisingdinitrogen monoxide obtainable by such a process as an oxidizing agentfor olefins.

EP 06 125 807.5 relates to a process for purifying a gas mixturecomprising dinitrogen monoxide, wherein absorption and desorption areeffected in aqueous solvent mixtures at particular pH values.

However, with the known processes, small amounts of oxygen which remainin the dinitrogen monoxide can be removed only with difficulty.Specifically traces of oxygen may, however, lead to undesiredby-products in subsequent reactions.

For example, in the oxidation of cyclopentene or cyclododecatriene, ithas been observed that between 1 and 4 mol of the olefins used areconsumed unproductively per mole of oxygen in the dinitrogen monoxideused, i.e. the presence of oxygen in the dinitrogen monoxide can lead tothe formation of by-products, for example to the formation of deposits,which can then lead to a blockage of the rector.

Proceeding from this prior art, it was an object of the presentinvention to provide a process with which the content of oxygen indinitrogen monoxide-containing streams can be reduced effectively andinexpensively. Dinitrogen monoxide purified in this way is requiredespecially as an oxidizing agent.

BRIEF SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention by a processfor purifying a gas mixture comprising dinitrogen monoxide, at leastcomprising the steps of:

-   -   (I) at least partially condensing a gas mixture G-I comprising        dinitrogen monoxide to obtain a liquid composition C-1        comprising dinitrogen monoxide,    -   (II) contacting the composition C-1 with a gas mixture M-1 to        obtain a composition C-2 and a gas mixture M-2.

DETAILED DESCRIPTION OF THE INVENTION

One advantage of the process according to the invention is that smalltraces of oxygen in particular can be removed from the gas mixturecomprising dinitrogen monoxide.

The term “gas mixture” as used in the context of the present inventionrefers to a mixture of two or more compounds which are in the gaseousstate at ambient pressure and ambient temperature. At alteredtemperature or altered pressure, the gas mixture may also be present inanother state of matter, for example liquid, and is still referred to asa gas mixture in the context of the present invention.

In the context of the present invention, the composition of the gasmixtures or of the liquefied gas mixtures, unless explicitly statedotherwise, is specified in % by volume. The data relate to thecomposition of the gas mixtures at ambient pressure and ambienttemperature.

In principle, the composition of the mixtures may be determined in thecontext of the present invention in any way known to those skilled inthe art. In the context of the present invention, the composition of thegas mixtures is preferably determined by gas chromatography. However, itmay also be determined by means of UV spectroscopy, IR spectroscopy orby wet chemical methods.

In the context of the present invention, a condensation of the gasmixture G-I is performed in step (I).

This affords a liquid composition C-1 comprising dinitrogen monoxide. Inthe at least partial condensation in step (I), it is additionallypossible for an uncondensed portion to be obtained, i.e. a gas mixtureG-K.

The process according to the invention further comprises a step (II)wherein the composition C-1 is contacted with a gas mixture M-1 toobtain a composition C-2 and a gas mixture M-2.

The gas mixture G-I may in principle originate from any desired source.For instance, it may be the product of a dinitrogen monoxide synthesisor an offgas stream of another process, which has been concentrated ifappropriate.

The condensation in step (I) of the process according to the inventioncan in principle be effected by any suitable process known to thoseskilled in the art. In the context of the present invention, the gasmixture G-I is at least partially condensed. According to the invention,from 20 to 99% by weight, preferably from 50 to 90% by weight and mostpreferably from 60 to 80% by weight of the gas mixture G-I is condensed.

In a further embodiment, the present invention therefore relates to aprocess for purifying a gas mixture comprising dinitrogen monoxide asdescribed above, wherein from 20 to 90% by weight of the gas mixture G-Iis condensed in step (I).

The treatment in step (I) of the process according to the inventionaffords the liquid composition C-1 in which the proportion of disruptivesecondary components, especially oxygen, has been reduced furthercompared to the gas mixture G-I.

According to the invention, the conditions are especially selected suchthat dinitrogen monoxide condenses, while the undesired constituents ofthe gas mixture G-I are condensed only to a minor degree, if at all.

At the same time, in the case of partial condensation, a gaseous mixtureG-K is obtained which, as well as dinitrogen monoxide, may comprisefurther components such as oxygen, nitrogen, carbon dioxide, argon orcarbon monoxide.

According to the invention, the gaseous mixture G-K comprises, forexample, from 70 to 90% by volume of dinitrogen monoxide, especiallyfrom 75 to 85% by volume, more preferably from 78 to 82% by volume.According to the invention, the gaseous mixture G-K further comprises,for example, from 4 to 18% by volume of carbon dioxide, especially from6 to 16% by volume and more preferably from 8 to 12% by volume of CO₂.The gaseous mixture G-K further comprises, for example, from 0.01 to 5%by volume of oxygen, especially from 0.5 to 3% by volume and morepreferably from 1.0 to 2.0% by volume of oxygen, and, for example, from0 to 1% by volume of argon, where the sum of the components of thegaseous mixture G-K adds up to 100% by volume.

Preferably in step (I), the gas mixture G-I is first compressed and thencooled, preferably in two stages. The gas mixture G-I is advantageouslycompressed to a pressure of from 1 to 35 bar, preferably from 2 to 30bar, more preferably from 3 to 27 bar. Cooling is preferably effected intwo stages, in which case cooled to from 1 to 25° C., preferably to from8 to 12° V, in the first stage and readily condensable constituents suchas water or organic solvents are removed, and then cooled, in the secondstage, preferably to from 0 to −70° C., more preferably from −1 to −30°C., especially from −2 to −25° C.

The liquid composition C-1, as well as dinitrogen monoxide,advantageously also comprises carbon dioxide. CO₂ has inertizing actionand ensures safe operation in the course of processing and especially inthe course of storage and further use of the liquid composition C-1. Ithas been found that, in the case of presence of CO₂ as an inert gas incompositions comprising N₂O, significantly smaller amounts of carbondioxide are required compared to other inert gases in order to preventthe self-decomposition capability of dinitrogen monoxide. Small amountsof CO₂ are therefore sufficient for inertization of the liquidcomposition C-1.

According to the invention, the process for purifying a gas mixturecomprising dinitrogen monoxide, as well as steps (I) and (II), may alsocomprise further steps. For instance, it is also possible that theprocess comprises further steps after step (I) and before step (II).

For example, in the process according to the invention, the compositionC-1 can be treated further. In the context of the present invention, itis more particularly possible that there is a further step forconcentration of the composition C-1. In principle, all suitable methodsknown to those skilled in the art for further concentration of thecomposition C-1 or for removal of impurities, for example of residues ofsolvent, are possible.

According to the invention, the process comprises especially a furtherstep (II) for removal of impurities from the composition C-1.Preferably, in step (II), the composition C-1 comprising dinitrogenmonoxide is contacted with a gas mixture M-1 to obtain a composition C-2and a gas mixture M-2.

By means of the treatment in step (II) of the process according to theinvention, it is possible to remove further impurities which might bedisruptive in a further reaction, for example oxygen, from the liquidcomposition C-1.

The gas mixture M-1 used may in principle be all substances which have alower boiling point than dinitrogen monoxide or mixtures thereof.Preference is given to using gases which do not react with dinitrogenmonoxide, for example nitrogen, helium, neon, argon, krypton, xenon,hydrogen, carbon monoxide, methane and tetrafluoromethane.

Particular preference is given to using nitrogen as gas mixture M-1.

In a further embodiment, the present invention therefore also relates toa process as described above for purifying a gas mixture comprisingdinitrogen monoxide, wherein the gas mixture M-1 is selected from thegroup consisting of nitrogen, helium, neon, argon, krypton, xenon,hydrogen, carbon monoxide, methane and tetrafluoromethane.

For the treatment in step (II), in the context of the present invention,it is possible to use any apparatus suitable for contacting gases andliquids with one another. The examples here include bubble columns, forexample operated in cocurrent or countercurrent, with or without randompacking or structured packing, in trickle or liquid-phase mode, stirredtanks, for example with sparging stirrers, or the like. The treatment instep (II) can be effected either batchwise or continuously. Preferenceis given to performing it continuously.

In a further embodiment, the present invention therefore also relates toa process as described above for purifying a gas mixture comprisingdinitrogen monoxide, wherein step (II) is performed continuously.

In the context of the present invention, step (II) is especiallyperformed in a bubble column, in which case the bubble column is morepreferably operated in countercurrent and is especially preferablyprovided with a structured packing.

The present invention therefore relates, in a further embodiment, alsoto a process as described above for purifying a gas mixture comprisingdinitrogen monoxide, wherein step (II) is performed in a bubble column.

In a further embodiment, the present invention also relates to a processas described above for purifying a gas mixture comprising dinitrogenmonoxide, wherein the bubble column is operated in countercurrent and ismore preferably provided with a structured packing.

The process is especially conducted in such a way that, in thecountercurrent bubble column, the composition C-1 is introduced at thetop and the composition C-2 is withdrawn at the bottom.

The treatment in step (II) is preferably performed at a temperaturebetween −90° C. and +37° C., preferably at a temperature between −80° C.and 0° C. Preference is given to performing the treatment in step (II)at a pressure which is at least as high as the vapor pressure of theliquid composition C-1 at the selected temperature and at not more than100 bar.

According to the invention, the amount of gas mixture M-1 used must besufficiently great to achieve the desired oxygen depletion but, on theother hand, as small as possible in order to avoid losses of dinitrogenmonoxide. Typically between 5 and 100 mol of gas mixture M-1 are usedper mole of oxygen in the liquid composition C-1, preferably between 15and 30 mol of gas mixture M-1 per mole of oxygen in the liquidcomposition C-1.

In step (II), a liquid composition C-2 is obtained, whose oxygen contenthas been reduced further compared to the liquid composition C-1.

According to the invention, the composition C-2 comprises, for example,from 75 to 95% by volume of dinitrogen monoxide, especially from 80 to90% by volume, more preferably form 82 to 88% by volume. According tothe invention, the composition C-2 further comprises, for example, from4 to 18% by volume of carbon dioxide, especially from 6 to 16% by volumeand more preferably from 8 to 12% by volume of CO₂. The composition C-2further comprises, for example, from 0.01 to 1.0% by volume of oxygen,especially from 0.05 to 0.5% by volume and more preferably from 0.1 to0.4% by volume of oxygen, and, for example, from 0 to 1% by volume ofnitrogen, where the sum of the components of the composition C-2 adds upto 100% by volume.

In step (II), a gas mixture M-2 is also obtained, which, in addition tothe gas mixture M-1, may comprise further components, for exampleoxygen.

According to the invention, the gas mixture M-2 comprises, for example,from 70 to 90% by volume of dinitrogen monoxide, especially from 75 to85% by volume, more preferably from 77 to 82% by volume. According tothe invention, the gas mixture M-2 additionally comprises, for example,from 4 to 18% by volume of carbon dioxide, especially from 6 to 16% byvolume and more preferably from 8 to 12% by volume of CO₂. The gasmixture comprises, for example, from 4 to 18% by volume of nitrogen,especially from 6 to 16% by volume and more preferably from 8 to 12% byvolume of nitrogen. The gas mixture M-2 further comprises, for example,from 0.01 to 5% by volume of oxygen, especially from 0.5 to 3% by volumeand more preferably from 1.0 to 2.0% by volume of oxygen, and, forexample, from 0 to 1% by volume of argon, where the sum of thecomponents of gas mixture M-2 adds up to 100% by volume.

In principle, the gas mixture G-I, in the context of the presentinvention, may originate from any desired source. According to theinvention, it is, however, preferred that the gas mixture G-I is a gasmixture comprising dinitrogen monoxide which has been concentratedbeforehand, for example by a process comprising an absorption anddesorption in a suitable solvent.

The present invention therefore relates, in a further embodiment, alsoto a process for purifying a gas mixture comprising dinitrogen monoxideas described above, wherein the gas mixture G-I is obtained by a processcomprising the steps of:

-   -   (A) treating a gas mixture G-0 comprising dinitrogen monoxide to        obtain a gas mixture G-A, at least comprising the steps of        -   (i) absorbing the gas mixture G-0 in a solvent mixture S-I            to obtain an offgas stream and a composition C-A        -   (ii) desorbing a gas mixture G-1 from the composition C-A to            obtain a solvent mixture S-I′.

When step (ii) of step (A) is performed directly before step (I) of theprocess according to the invention, the composition of the gas mixtureG-1 corresponds to that of the gas mixture G-I.

In step (A), a gas mixture G-0 comprising dinitrogen monoxide is treatedto obtain a gas mixture G-A, step (A) comprising at least steps (i) and(ii). In step (i), the gas mixture G-0 is absorbed in a solvent mixtureS-I to obtain an offgas stream and a composition C-A. In step (ii), agas mixture G-1 is desorbed from the composition C-A to obtain a solventmixture S-I′.

In the context of the present invention, the gas mixture G-0 is a gasmixture comprising dinitrogen monoxide, which is used in the processaccording to the invention. The gas mixture G-0 may comprise furthercomponents as well as dinitrogen monoxide.

According to the invention, the gas mixture G-0 comprising dinitrogenmonoxide used may in principle stem from any source.

When a gas mixture G-0 is used, its content of dinitrogen monoxide issubstantially arbitrary, as long as it is guaranteed that the inventivepurification is possible.

The N₂O-containing gas mixtures which are used as gas mixture G-0 forthis process generally have an N₂O content between 2 and 80% by volumeof N₂O. It further comprises, for example, from 2 to 21% by volume of O₂and up to 30% by volume of NO_(x) as undesired components. In addition,it may also comprise varying amounts of N₂, H₂, CO₂, CO, H₂O, NH₃;traces of organic compounds may also be present. For example, the gasmixture G-0 may also comprise from 9 to 13% by volume of N₂ and up to5.5% by volume of NH₃. The sum of the components of the gas mixture G-0adds up to 100% by volume.

In a preferred embodiment of the process according to the invention, agas mixture G-0 comprising at least 3% by volume of dinitrogen monoxideis used, but preference is given in turn to using mixtures having adinitrogen monoxide content in the range from 4 to 60% by volume, morepreferably in the range from 5 to 25% by volume and especiallypreferably in the range from 8 to 14% by volume.

In this embodiment, the gas mixture G-0 preferably has an N₂O content offrom 8 to 18% by volume, more preferably, for example, 9% by volume, 10%by volume, 11% by volume, 12% by volume, 13% by volume, 14% by volume,15% by volume, 16% by volume or 17% by volume.

The gas mixture G-0 has, for example, a CO₂ content of from 0.1 to 7.5%by volume, preferably from 0.5 to 5% by volume, more preferably from 1to 2.5% by volume. At the same time, the gas mixture G-0 has, forexample, an O₂ content of from 1 to 10% by volume, preferably from 2 to7.5% by volume, more preferably, for example, from 3.0 to 6% by volume.In addition, the gas mixture G-0 may also comprise from 50 to 95% byvolume of N₂, preferably from 60 to 90% by volume, more preferably from70 to 85% by volume, and also further components, for example nitrogenoxides or solvent residues. NO_(x) may, for example, be present in anamount of from 0 to 0.2% by volume, preferably from 0.0001 to 0.15% byvolume, more preferably from 0.0005 to 0.1% by volume. The sum of thecomponents of the gas mixture G-0 adds up to 100% by volume.

In a preferred embodiment of the present invention, the gas mixture G-0comprising dinitrogen monoxide is at least one dinitrogenmonoxide-containing offgas of a chemical process. The scope of thepresent invention also embraces embodiments in which at least twonitrogen monoxide-containing offgases of a single plant serve as the gasmixture comprising dinitrogen monoxide. Equally embraced are embodimentsin which at least one dinitrogen monoxide-containing offgas of one plantand at least one further dinitrogen monoxide-containing offgas of atleast one further plant serve as the gas mixture comprising dinitrogenmonoxide.

Accordingly, the present invention also relates to a process asdescribed above, wherein the gas mixture comprising dinitrogen monoxideis at least one dinitrogen monoxide-containing offgas of at least oneindustrial process.

The term “gas mixture comprising dinitrogen monoxide” refers in thecontext of the present invention both to embodiments in which the offgasmentioned is subjected to the inventive purification process inunmodified form and to embodiments in which at least one of the offgasesmentioned is subjected to a modification.

The term “modification” as used in this context within the scope of thepresent invention refers to any suitable process by which the chemicalcomposition of a gas mixture is altered. Accordingly, the term“modification” comprises, inter alia, embodiments in which a dinitrogenmonoxide-containing offgas is concentrated with regard to the dinitrogenmonoxide content in at least one suitable process. Preference is givento not subjecting the offgas to any modification.

In a further embodiment, the chemical composition of an offgas may alsobe altered by adding pure dinitrogen monoxide to the offgas.

The gas mixture G-0 comprising N₂O which is used may, for example, be anoffgas from an industrial process. It preferably stems from an offgas ofa plant for oxidation of alcohols, aldehydes or ketones with nitricacid, for example from an adipic acid plant, dodecanedicarboxylic acidplant or glyoxal plant, from the offgas of a nitric acid plant whichuses the above offgas streams as a reactant, from the offgas of a plantfor the partial oxidation of NH₃ or from the offgas of a plant whichuses the gas mixtures generated therein, for example a hydroxylamineplant.

According to the invention, it is also possible to use a mixture ofdifferent offgases.

In a more preferred embodiment of the present invention, the at leastone dinitrogen monoxide-containing offgas stems from an adipic acidplant, a dodecanedicarboxylic acid plant, a glyoxal plant, ahydroxylamine plant and/or a nitric acid plant, the latter in turnpreferably being operated with at least one offgas of an adipic acidplant, of a dodecanedicarboxylic acid plant or of a glyoxal plant.

In a preferred embodiment, the offgas stream of an adipic acid plant isused, in which generally from 0.8 to 1.0 mol of N₂O per mole of adipicacid formed is formed by oxidation of cyclohexanol/cyclohexanonemixtures with nitric acid. As described, for example, in A. K. Uriarteet al., Stud. Surf. Sci. Catal. 130 (2000) p. 743-748, the offgases ofadipic acid plants also comprise different concentrations of furtherconstituents including nitrogen, oxygen, carbon dioxide, carbonmonoxide, nitrogen oxides, water and volatile organic compounds.

The abovementioned dodecanedicarboxylic acid plant is substantially ofan identical plant type.

An example of a typical composition of an offgas of an adipic acid plantor of a dodecanedicarboxylic acid plant is reproduced in the followingtable:

Component Concentrations % by weight NO_(x) 19-25 N₂O 20-28 N₂ 30-40 O₂ 7-10 CO₂ 2-3 H₂O ~ 7

The offgas stream of an adipic acid plant or of a dodecanedicarboxylicacid plant may be used directly in the process according to theinvention.

In a likewise preferred embodiment, the offgas stream of a nitric acidplant is used which is fed fully or partly with offgases comprisingdinitrogen monoxide and nitrogen oxides from other processes. In suchnitric acid plants, nitrogen oxides are adsorbed and for the most partconverted to nitric acid, while dinitrogen monoxide is not converted.For example, such a nitric acid plant may be supplied by nitrogen oxideswhich are prepared by selective combustion of ammonia and by offgases ofan adipic acid plant and/or by offgases of a dodecanedicarboxylic acidplant. It is equally possible to supply such a nitric acid plant solelyby offgases of an adipic acid plant and/or by offgases of adodecanedicarboxylic acid plant.

The offgases of such nitric acid plants always comprise varyingconcentrations of still further constituents including nitrogen, oxygen,carbon dioxide, carbon monoxide, nitrogen oxides, water and volatileorganic compounds.

An example of a typical composition of an offgas of such a nitric acidplant is reproduced in the following table:

Component Concentrations % by weight NO_(x) <0.1 N₂O  4-36 N₂ 57-86 O₂3-9 CO₂ 1-4 H₂O ~ 0.6

The offgas stream of a nitric acid plant of this type may be useddirectly in the process according to the invention.

In a likewise preferred embodiment of the process according to theinvention, the offgas stream of a hydroxylamine plant is used, in which,for example, ammonia is initially oxidized with air or oxygen to giveNO, and small amounts of dinitrogen monoxide are formed as a by-product.The NO is subsequently hydrogenated with hydrogen to give hydroxylamine.Since dinitrogen monoxide is inert under the hydrogenation conditions,it accumulates in the hydrogen circuit. In preferred process versions,the purge stream of a hydroxylamine plant comprises dinitrogen monoxidein the range from 9 to 13% by volume in hydrogen. This purge stream maybe used as such for the inventive purification. It is equally possibleto concentrate this stream in a suitable manner with regard to thedinitrogen monoxide content as described above.

Accordingly, the present invention also relates to a process asdescribed above, wherein the gas mixture G-0 is the offgas of an adipicacid plant and/or of a dodecanedicarboxylic acid plant and/or of aglyoxal plant and/or of a hydroxylamine plant and/or of a nitric acidplant operated with the offgas of an adipic acid plant and/or of adodecanedicarboxylic acid plant and/or of a glyoxal plant.

It is equally possible in the context of the process according to theinvention to selectively prepare dinitrogen monoxide for use in theprocess. Preference is given inter alia to the preparation via thethermal decomposition of NH₄NO₃, as described, for example, in U.S. Pat.No. 3,656,899. Preference is likewise further given to the preparationvia the catalytic oxidation of ammonia, as described, for example, inU.S. Pat. No. 5,849,257 or in WO 98/25698.

In the absorption in step (i), the gas mixture G-0 is absorbed in asolvent mixture S-I. In the context of the present invention, it ispossible in principle to use any method of absorption known to thoseskilled in the art. This affords an offgas stream and a composition C-A.The composition C-A is then treated further in step (ii). The gasmixture G-1 is desorbed from the composition C-A to obtain a solventmixture S-I′.

According to the invention, the gas mixture G-1 comprises at leastdinitrogen monoxide and may comprise further components.

According to the invention, the solvent mixture S-I used may be anysuitable solvent mixture known to those skilled in the art, providedthat it is ensured that the gas mixture G-0, especially dinitrogenmonoxide, is at least partly absorbed.

In step (A), a gas mixture G-A comprising dinitrogen monoxide isobtained. The gas mixture G-A may additionally comprise furthercomponents. When step (A) does not comprise any further steps after step(ii), the composition of the gas mixture G-1 is identical to that of thegas mixture G-A.

In step (I), the gas mixture G-I obtained from step (A) is at leastpartly condensed to obtain a liquid composition C-1 comprisingdinitrogen monoxide and, if appropriate, a gaseous mixture G-K. In thecontext of the present invention, the liquid composition C-1 comprisesdinitrogen monoxide and may comprise further components.

According to the invention, the gaseous mixture G-K comprises, forexample, from 70 to 90% by volume of dinitrogen monoxide, especiallyfrom 75 to 85% by volume, more preferably from 78 to 82% by volume.According to the invention, the gaseous mixture G-K additionallycomprises, for example, from 4 to 18% by volume of carbon dioxide,especially from 6 to 16% by volume and more preferably from 8 to 12% byvolume of CO₂. The gaseous mixture G-K further comprises, for example,from 0.01 to 5% by volume of oxygen, especially from 0.5 to 3% by volumeand more preferably from 1.0 to 2.0% by volume of oxygen, and, forexample, from 0 to 1% by volume of argon, where the sum of thecomponents of the gaseous mixture G-K adds up to 100% by volume.

According to the invention, the process may comprise further steps. Forexample, it is possible in the context of the present invention thatfurther steps are included between steps (A) and (I).

According to the invention, step (A) may also comprise further steps.More particularly, it is possible that step (A) comprises a furtherabsorption of the gas mixture G-1 in a suitable solvent mixture and afurther desorption.

In a further embodiment, the present invention therefore relates to aprocess as described above for purifying a gas mixture comprisingdinitrogen monoxide, wherein step (A) additionally comprises steps (iii)and (iv):

-   -   (iii) absorbing the gas mixture G-1 in a solvent mixture S-II to        obtain an offgas stream and a composition C-B    -   (iv) desorbing a gas mixture G-2 from the composition C-B to        obtain a solvent mixture S-II′.

According to the invention, the solvent mixture S-II used may be anysuitable solvent mixture known to those skilled in the art, providedthat it is ensured that the gas mixture G-1, especially dinitrogenmonoxide, is at least partly absorbed.

When step (A) does not comprise any further steps after step (iv), thecomposition of gas mixture G-2 is identical to that of gas mixture G-I.

In the context of the present invention, it is also possible that step(A), as well as steps (i) and (ii), or as well as steps (i), (ii), (iii)and (iv), comprises further steps, including further absorptions anddesorptions.

For instance, it is possible in the context of the present inventionthat the process comprises a plurality of steps (i) and (ii) or aplurality of steps (iii) and (iv).

In a further embodiment, the present invention relates to a process asdescribed above for purifying a gas mixture comprising dinitrogenmonoxide, wherein step (A) comprises further steps.

In a preferred embodiment, the process according to the inventioncomprises, in step (A), at least the steps (i) and (ii), and, in afurther embodiment, also steps (iii) and (iv), wherein the solventmixtures S-I and S-II are used.

According to the invention, the solvent mixtures S-I and/or S-II usedmay be any suitable solvent mixture known to those skilled in the art,provided that it is ensured that especially dinitrogen monoxide isabsorbed.

Suitable solvent mixtures S-I and S-II for the absorption in step (i) or(iii) of step (A) are those which have a better solubility for N₂O andpreferably also CO₂ as an inert component than for the undesiredcomponents of the incoming reactant gas G-0.

According to the invention, the solvent mixtures S-I and/or S-II usedmay be organic solvents or aqueous solvent mixtures. In a furtherembodiment, the present invention therefore relates to a process asdescribed above for purifying a gas mixture comprising dinitrogenmonoxide, wherein the solvent mixture S-I or the solvent mixture S-II orthe solvent mixture S-I and the solvent mixture S-II is/are selectedfrom the group consisting of organic solvents and aqueous solventmixtures.

According to the invention, the organic solvents used may be anysolvents in which the ratio between N₂O solubility (in mol/mol ofsolvent) and the solubility of the undesired secondary components underthe conditions existing in the absorber (this ratio is referred tohereinafter as γ) is at least 5. This ratio may be determined for eachindividual component present in the gas mixture. Preferred organicsolvents have, for example at 30° C., a γ_(O2) value of from 6 to 30,preferably from 9 to 25, and a γ_(N2) value of greater than 10,preferably of greater than 15, in particular of greater than 20.

Examples of suitable organic solvents are, for example, aliphatichydrocarbons, preferably having at least 5 carbon atoms, more preferablyhaving at least 8 carbon atoms, substituted or unsubstituted aromatichydrocarbons, esters, ethers, amides, lactones, lactams, nitriles, alkylhalides, olefins or mixtures of these solvents.

According to the invention, preference is given to solvents which have aboiling point at standard pressure of at least 100° C., since thisreduces the solvent losses both in the offgas stream of the absorber andof the desorber.

In addition, solvents suitable in accordance with the inventionsimultaneously have a good solubility for dinitrogen monoxide. Thesolubility is specified by the ratio between the partial pressure of N₂Oin the gas phase and the molar proportion of N₂O in the liquid phase(Henry coefficient, H_(N2O)), i.e. a small value means a high solubilityof dinitrogen monoxide in the solvent. This ratio for an organic solventused in the first step at 30° C. is preferably less than 1000, morepreferably less than 750, particularly preferably less than 500, inparticular less than 150.

Suitable solvents also include N-methylpyrrolidone, dimethylformamide,dimethyl sulfoxide, propylene carbonate, sulfolane,N,N-dimethylacetamide or cyclopentane. Particular preference is given inthe context of the present invention, for example, to toluene,nitrobenzene, 1,2-dichlorobenzene, tetradecane, for example atechnical-grade mixture of saturated hydrocarbons having predominantly14 carbon atoms, and dimethyl phthalate.

In a preferred embodiment, the present invention therefore relates to aprocess for purifying a gas mixture comprising dinitrogen monoxide asdescribed above, wherein the organic solvent is selected from the groupconsisting of toluene, nitrobenzene, 1,2-dichlorobenzene, tetradecaneand dimethyl phthalate.

According to the invention, it is likewise possible to use aqueoussolvent mixtures as solvent mixture S-I and/or S-II. In principle, theabove remarks apply for the suitability of the aqueous solvent mixturesfor the process according to the invention. In particular, the solventmixtures S-I and/or S-II used may be solvent mixtures at leastcomprising 50% by weight of water based on the overall solvent mixture.It is also possible in the context of the present invention that the pHof the solvent mixture used is set within a particular range. Accordingto the invention, a suitable pH for an aqueous solvent mixture is, forexample, in the range from 2.0 to 8.0. It is also possible in accordancewith the invention that the pH of the aqueous solvent mixtures S-I orS-II used in the individual absorption steps is varied.

In the context of this application, the pH is measured with acommercially available glass electrode which has been calibratedbeforehand against a buffer of known pH. All pH data are based on ameasurement with a calibrated and temperature-compensated glasselectrode. If the calibration temperature differs from the measurementtemperature, a temperature compensation is used. This definition andthis method correspond to the currently valid IUPAC recommendation (R.P. Buck et al., Pure Appl. Chem. (2002) 74(11), p. 2169-2200 andespecially section 11 thereof).

Water has a high selectivity for the desired components, especiallydinitrogen monoxide and carbon dioxide. At the same time, the absolutesolubility of dinitrogen monoxide in water is sufficient to achievefurther concentration. Water as a solvent has the advantage that, evenunder pressure in the presence of concentrated dinitrogen monoxide, nosafety problems occur. At the same time, no contamination of the gasmixture obtained from the desorption with an organic solvent can occur,which would necessitate additional purification steps.

According to the invention, both solvent mixture S-I and S-II may be anorganic solvent mixture or an aqueous solvent mixture. According to theinvention, it is possible that the solvent mixture S-I used is anorganic solvent and the solvent mixture S-II used is an aqueous solventmixture. It is equally possible that the solvent mixture S-I used is anaqueous solvent mixture and the solvent mixture S-II an organic solvent.In the context of the present invention, both solvent mixture S-I andsolvent mixture S-II are preferably an aqueous solvent mixture.

It is additionally preferred that, when the solvent mixture S-I and/orS-II used is an aqueous solvent mixture, the pH of the aqueous solventmixture is set within a particular range.

By virtue of the inventive selection of the pH of the solvent mixtureS-I and solvent mixture S-II, almost complete depletion of NO_(x) isachieved. This makes a separate removal of NO_(x), for example by meansof DeNOx or SCR-DeNOx, superfluous. As a result, in the processaccording to the invention, there is, for example, also no risk ofcontamination of the product stream with NH₃, which is used as areducing agent for the DeNOx stage.

As a result of the controlled selection, which is preferred inaccordance with the invention, of the pH of the solvent mixture S-I andof the solvent mixture S-II, it is possible especially to achieve gooddepletion of NO_(x) with only a minimal change in the carbon dioxidecontent.

The solvent mixtures S-I and S-II used in accordance with the inventionhave, at the pH preferred in accordance with the invention, a highselectivity for the desired components, especially dinitrogen monoxideand carbon dioxide. At the same time, the absolute solubility ofdinitrogen monoxide in the solvent mixture S-I or S-II used inaccordance with the invention is sufficient to achieve concentration.The solvent mixture S-I or S-II used in accordance with the inventionhas the advantage that, even under pressure in the presence ofconcentrated dinitrogen monoxide, no safety problems occur.

According to the invention, the pH of the aqueous solvent mixture in theabsorption may preferably be in the range from 3.5 to 8.0. At this pH,according to the invention, there is a good absorption of dinitrogenmonoxide and carbon dioxide in the solvent mixture, while other gaseswhich may be present in the gas mixture G-0 are absorbed to a smalldegree, if at all. The pH is preferably within a range from 5.0 to 7.5,more preferably within a range from 6.0 to 7.0.

According to the invention, the pH is measured before or during thecontacting of the gas mixture with the aqueous solvent mixture and then,for example, the pH is adjusted by suitable measures. It is equallypossible in accordance with the invention that no measures are needed toadjust the pH.

In principle, the pH can, in accordance with the invention, be adjustedby all measures known to those skilled in the art. Suitable measures foradjusting the pH are, for example, addition of an acid or alkali oraddition of further solvents.

For example, the pH of the aqueous solvent mixture is measured before orafter the absorption and the pH is set within the inventive range bysuitable measures. According to the invention, the pH can be measuredcontinuously or discontinuously.

When the pH values of solvent mixture S-I and of solvent mixture S-IIare adjusted, the pH of solvent mixture S-I and of solvent mixture S-IIcan be adjusted independently of one another. According to theinvention, it is also possible that only the pH of solvent mixture S-Ior of solvent mixture S-II is adjusted. However, it is also possible inaccordance with the invention for the pH of solvent mixture S-I and ofsolvent mixture S-II to be adjusted within the same range.

In the context of the present invention, an aqueous solvent mixture isunderstood to mean a solvent mixture at least comprising 50% by weightof water, for example from 50 to 100% by weight of water, preferably atleast 60% by weight of water, especially at least 70% by weight ofwater, more preferably at least 80% by weight of water, for example atleast 90% by weight of water. The aqueous solvent mixture preferablycomprises at least 90% by weight of water, based in each case on theoverall aqueous solvent mixture.

The present invention therefore also relates to a process as describedabove for purifying a gas mixture comprising dinitrogen monoxide,wherein the solvent mixture S-I or the solvent mixture S-II or thesolvent mixture S-I and the solvent mixture S-II comprise(s) at least90% by weight of water, based in each case on the overall solventmixture.

According to the invention, the aqueous solvent mixture, in addition towater, may also comprise other polar water-miscible solvents, forexample glycols. In addition, the aqueous solvent mixture, as well aswater, may also comprise dissolved salts, for example salts of thealkali metals or alkaline earth metals, especially hydroxides,hydrogencarbonates, carbonates, nitrates, nitrites, sulfates,hydrogenphosphates or phosphates.

According to the invention, the content of salts in the aqueous solventmixture is less than 5% by weight, preferably less than 2.5% by weight,especially less than 2.0% by weight. The content of salts in the aqueoussolvent mixture is, for example, from 0.0001 to 5% by weight, preferablyfrom 0.001 to 2.5% by weight, especially from 0.01 to 2.0% by weight.

According to the invention, the content of salts in the aqueous solventmixture is preferably controlled by continuously or discontinuouslyreplacing a portion of the solvent mixture laden with salts with anappropriately adjusted amount of fresh solvent mixture.

According to the invention, the pH of the aqueous solvent mixture can beadjusted by means of any method known to those skilled in the art. Moreparticularly, the pH can be adjusted by adding a base to the aqueoussolvent mixture.

In principle, the base used may be any conceivable compound whose pH, asa 1% by weight solution in water, is >8.0. Preference is given inaccordance with the invention to using strong inorganic bases,especially hydroxides, carbonates, hydrogen-carbonates or phosphates ofalkali metals or alkaline earth metals. Particular preference is givento using NaOH, KOH, Na₂CO₃, NaHCO₃, Na₃PO₄, K₃PO₄. Additionallypreferred is the use of the bases in the form of a concentrated aqueoussolution.

In the context of the present invention, suitable concentration rangesare, for example, from 10 to 60% aqueous solutions, preferably from 20to 55% aqueous solutions, more preferably from 25 to 50% aqueoussolutions, for example 30% aqueous solutions, 35% aqueous solutions, 40%aqueous solutions, 45% aqueous solutions or 50% aqueous solutions.

Particular preference is given in accordance with the invention to theuse of an aqueous NaOH solution as the base.

In a preferred embodiment of the present invention, the base used is afrom 25 to 50% aqueous NaOH solution.

For example, the pH of the aqueous solvent mixture is adjusted by addinga base selected from the group consisting of alkali metal hydroxides,alkali metal carbonates, alkali metal hydrogencarbonates, alkali metalphosphates, alkaline earth metal hydroxides, alkaline earth metalcarbonates, alkaline earth metal hydrogencarbonates and alkaline earthmetal phosphates.

In step (i), according to the invention, there is an at least partialabsorption of the gas mixture G-0 in a solvent mixture S-I to obtain acomposition C-A and an offgas stream depleted of the absorbed gases.

In the context of the present invention, a depleted offgas stream isunderstood to mean a gas stream which comprises the gases not absorbedin the absorption in the solvent mixture S-I or S-II.

The composition C-A comprises the solvent mixture S-I and the gasesabsorbed therein.

When the solvent mixture S-I used is water, the composition C-Acomprises, for example, from 90.0 to 99.9999% by weight of water,especially from 95.0 to 99.999% by weight and preferably from 98.0 to99.99% by weight of water; for example from 0.01 to 0.25% by weight ofdinitrogen monoxide, especially from 0.05 to 0.2% by weight andpreferably from 0.1 to 0.15% by weight of dinitrogen monoxide; forexample from 0.0001 to 0.1% by weight of carbon dioxide, especially from0.001 to 0.05% by weight of carbon dioxide; for example from 0.0001 to0.1% by weight of nitrogen, especially from 0.001 to 0.05% by weight ofnitrogen; for example from 0.05 to 1.5% by weight of sodium nitrite,especially from 0.1 to 1.0% by weight and preferably from 0.25 to 0.75%by weight of sodium nitrite; for example from 0.05 to 1.5% by weight ofsodium nitrate, especially from 0.1 to 1.0% by weight and preferablyfrom 0.25 to 0.75% by weight of sodium nitrate; for example from 0.0001to 0.1% by weight of sodium hydrogencarbonate, especially from 0.001 to0.05% by weight of sodium hydrogencarbonate; and traces of oxygen andargon. The sum of the components of composition C-A adds up to 100% byweight.

According to the invention, the depleted offgas stream comprises, forexample, from 0.1 to 2.0% by volume of argon, especially from 0.25 to1.5% by volume and preferably from 0.5 to 1.0% by volume of argon; forexample from 1.0 to 10% by volume of oxygen, especially from 2.5 to 7.5%by volume and preferably from 4.0 to 6.0% by volume of oxygen; forexample from 1.0 to 10% by volume of dinitrogen monoxide, especiallyfrom 2.5 to 7.5% by volume and preferably from 4.0 to 6.0% by volume ofdinitrogen monoxide; for example from 70 to 99.9% by volume of nitrogen,especially from 75 to 95% by volume and preferably from 80 to 90% byvolume of nitrogen; for example from 0.01 to 0.5% by volume of carbonmonoxide, especially from 0.05 to 0.25% by volume and preferably from0.08 to 0.1% by volume of carbon monoxide; for example from 0.1 to 1.5%by volume of carbon dioxide, especially from 0.25 to 1.0% by volume andpreferably from 0.5 to 0.75% by volume of carbon dioxide; for examplefrom 0.1 to 1.5% by volume of water, especially from 0.25 to 1.0% byvolume and preferably from 0.5 to 0.75% by volume of water. The sum ofthe components of the offgas stream adds up to 100% by volume.

Preference is given to performing step (i) of the process according tothe invention continuously. In the context of the present invention,this means that the solvent mixture S-I and the gas mixture G-0 arecontacted continuously, which continuously forms the composition C-A andthe depleted offgas stream.

According to the invention, in the absorption in step (i), preferablydinitrogen monoxide and carbon dioxide are absorbed. According to theinvention, it is also possible, for example, for nitrogen, oxygen andargon to be absorbed. Nitrogen oxides NO_(x) are also absorbed in step(i).

In a preferred embodiment, the process according to the inventionfurther comprises a step (ii) in which a gas mixture G-1 is desorbedfrom the composition C-A to obtain a solvent mixture S-I′.

In step (ii), preferably dinitrogen monoxide and carbon dioxide aredesorbed from the composition C-A.

As well as the solvent mixture S-I used, the solvent mixture S-I′ alsocomprises as yet undesorbed gases and conversion products.

For example, in the case that the solvent mixture S-I used with aparticular adjusted pH in the process according to the invention and thepH is adjusted by adding an alkali, especially sodium hydroxidesolution, the solvent mixture S-I′ comprises, in accordance with theinvention, for example from 90.0 to 99.9999% by weight of water,especially from 95.0 to 99.999% by weight and preferably from 98.0 to99.99% of water; for example from 0.001 to 0.1% by weight of dinitrogenmonoxide, for example from 0.05 to 1.5% by weight of sodium nitrite,especially from 0.1 to 1.0% by weight and preferably from 0.25 to 0.75%by weight of sodium nitrite; for example from 0.05 to 1.5% by weight ofsodium nitrate, especially from 0.1 to 1.0% by weight and preferablyfrom 0.25 to 0.75% by weight of sodium nitrate; for example from 0.0001to 0.1% by weight of sodium hydrogencarbonate, especially from 0.001 to0.05% by weight of sodium hydrogencarbonate. The solvent mixture S-I′may additionally also comprise further compounds. The sum of thecomponents of the solvent mixture S-I′ adds up to 100% by weight.

In the context of the present invention, the gas mixture G-1 has, forexample, an N₂O content of from 40 to 80% by volume, preferably from 45to 75% by volume, especially from 50 to 65% by volume, more preferably,for example, 51% by volume, 52% by volume, 53% by volume, 54% by volume,55% by volume, 56% by volume, 57% by volume, 58% by volume, 59% byvolume, 60% by volume, 61% by volume, 62% by volume, 63% by volume, 64%by volume or 65% by volume.

The gas mixture G-1 has, for example, a CO₂ content of from 5 to 15% byvolume, preferably from 6 to 12% by volume, more preferably, forexample, 7% by volume, 9% by volume, 10% by volume or 11% by volume. Atthe same time, the gas mixture G-1 has, for example, an O₂ content offrom 1.0 to 4.0% by volume, preferably from 1.5 to 3.5% by volume, morepreferably from 2.5 to 3.1% by volume, for example 2.6% by volume, 2.7%by volume, 2.8% by volume, 2.9% by volume or 3.0% by volume. Inaddition, the gas mixture G-1 may also comprise from 20 to 40% by volumeof N₂, preferably from 20 to 35% by volume, and also further components,for example nitrogen oxides. NO_(x) may be present, for example, in anamount of from 0 to 0.1% by volume, preferably from 0.0001 to 0.01% byvolume, more preferably from 0.0002 to 0.05% by volume. The sum of thecomponents of the gas mixture G-1 adds up to 100% by volume. The gasmixture G-1 may additionally comprise from 0 to 10% by volume of water,especially from 2 to 8% by volume and preferably from 4 to 6% by volumeof water.

According to the invention, step (A) may comprise further steps,especially a further absorption and desorption in a suitable solventaccording to steps (iii) and (iv). In steps (iii) and (iv), there is anabsorption of the gas mixture G-1 in a suitable solvent mixture S-II anda subsequent desorption of the gas mixture G-2.

In the absorption in step (iii), there is, according to the invention,an absorption in a solvent mixture S-II to obtain a composition C-B andan offgas stream depleted of the absorbed gases.

The composition C-B comprises the solvent mixture S-II and the gasesabsorbed therein.

When the solvent mixture S-II used is water, the composition C-Bcomprises, for example, from 90.0 to 99.9999% by weight of water,especially from 95.0 to 99.999% by weight and preferably from 98.0 to99.99% by weight of water; for example from 0.01 to 2.5% by weight ofdinitrogen monoxide, especially from 0.1 to 1.5% by weight andpreferably from 0.5 to 1.0% by weight of dinitrogen monoxide; forexample from 0.001 to 0.5% by weight of carbon dioxide, especially from0.01 to 0.25% by weight of carbon dioxide; for example from 0.0001 to0.1% by weight of nitrogen, especially from 0.001 to 0.05% by weight ofnitrogen; and traces of oxygen and argon. The sum of the components ofthe composition C-B adds up to 100% by weight.

Preference is given to performing step (iii) of the process according tothe invention continuously. In the context of the present invention,this means that the solvent mixture S-II and the gas mixture G-1 arecontacted continuously, which continuously forms the composition C-B andthe depleted offgas stream.

Preference is given to performing steps (i) and (iii) of the processaccording to the invention continuously.

According to the invention, in the absorption in step (iii), preferablydinitrogen monoxide and carbon dioxide are absorbed. Nitrogen oxidesNO_(x) remaining in the gas mixture G-1 are preferably also absorbed instep (iii).

According to the invention, preferably from 60 to 80% of the enteringgas stream are absorbed in step (iii).

In a preferred embodiment, the process according to the inventionpreferably further comprises a step (iv) in which a gas mixture G-2 isdesorbed from the composition C-B to obtain a solvent mixture S-II′.

In step (iv), preference is given to desorbing dinitrogen monoxide andcarbon dioxide from the composition C-B.

As well as the solvent mixture S-II used, the solvent mixture S-II′comprises as yet undesorbed gases and conversion products.

The resulting gas mixture G-2 comprises at least 50% by volume of N₂O,more preferably at least 60% by volume of N₂O and most preferably atleast 75% by volume of N₂O. Typically, gas mixture G-2 comprises up to99% by volume of N₂O, especially up to 97% by volume of N₂O, for exampleup to 96% by volume of N₂O, up to 95% by volume of N₂O, up to 94% byvolume of N₂O, up to 93% by volume of N₂O, up to 92% by volume of N₂O,up to 91% by volume of N₂O, up to 90% by volume of N₂O or else up to 85%by volume of N₂O.

In the context of the present invention, the gas mixture G-2 has, forexample, an N₂O content of from 60 to 95% by volume, preferably from 70to 90% by volume, especially from 75 to 85% by volume, more preferably,for example, 76% by volume, 77% by volume, 78% by volume, 79% by volume,80% by volume, 81% by volume, 82% by volume, 83% by volume, 84% byvolume or 85% by volume.

The gas mixture G-2 has, for example, a CO₂ content of from 1 to 20% byvolume, preferably from 5 to 15% by volume, more preferably, forexample, 6% by volume, 7% by volume, 8% by volume, 9% by volume, 10% byvolume, 11% by volume, 12% by volume, 13% by volume or 14% by volume. Atthe same time, the gas mixture G-2 has, for example, an O₂ content offrom 0.01 to 5.0% by volume, preferably from 0.1 to 2.5% by volume, morepreferably, for example, from 0.2 to 1.0% by volume. In addition, thegas mixture G-2 may also comprise from 0.1 to 10% by volume of N₂,preferably from 0.5 to 5% by volume, and also further components, forexample nitrogen oxides or solvent residues. At the same time, the gasmixture G-2 comprises less than 1% by volume of O₂, especially less than0.5% by volume of O₂, less than 0.5% by volume of NO_(x). NO_(x) may bepresent, for example, in an amount of from 0 to 0.1% by volume,preferably from 0.0001 to 0.01% by volume, more preferably from 0.0002to 0.02% by volume. The sum of the components of the gas mixture G-2adds up to 100% by volume.

When step (A) comprises no further steps after step (iv), thecomposition of gas mixture G-I corresponds to the composition of gasmixture G-2.

The absorption in step (i) or (iii) in step (A) of the process accordingto the invention can in principle be effected by all methods known tothose skilled in the art. More particularly, the absorption in thesolvent mixture can be brought about by increasing the pressure of thereactant gas or by lowering the temperature of the solvent mixture or bya combination of the measures stated.

In step (i) or (iii) of the process according to the invention,preference is given to first compressing the gas mixture, for example toa pressure of from 10 to 35 bar, preferentially from 13 to 30 bar,preferably from 14 to 25 bar. Subsequently, the compressed gas mixtureis preferably contacted at this pressure with the solvent mixture S-I instep (i) or in the solvent mixture S-II in step (iii).

The present invention therefore also relates to a process as describedabove for purifying a gas mixture G-0 comprising dinitrogen monoxide,wherein the pressure in the absorption in step (i) or (iii) or (i) and(iii) is within a range from 10 to 35 bar.

According to the invention, the absorption in step (i) and step (iii) iseffected in equipment (absorbers) in which a gas-liquid phase interfaceis generated, through which mass and heat transfer between the phases isenabled, and which are provided if required with internal or externalequipment for heat supply and/or heat removal.

The phases within the absorber can be conducted in cocurrent, incountercurrent, or in a combination thereof.

According to the invention, the absorption can be effected in one ormore stages, preferably in one stage. In the absorption, the absorberused is preferably a device with a plurality of theoretical plates,especially from 2 to 8 theoretical plates, more preferably from 3 to 6.

Possible embodiments of the absorber are in each case columns withtrays, for example bubble-cap trays or sieve trays, columns withstructured internals, for example structured packings, columns withunstructured internals, for example random packings, or apparatus inwhich the liquid phase is present in dispersed form, for example as aresult of spraying in nozzles, or a combination thereof.

The desorption of the gas mixture G-1 or G-2 from the composition C-A orcomposition C-B in step (ii) or (iv) of the process according to theinvention can be brought about by lowering the pressure over the solventmixture, increasing the temperature of the solvent mixture, or bystripping with solvent vapor, or a combination thereof.

The demands on the equipment (desorbers) for the desorption of the gasmixture G-1 or G-2 from the composition C-A or composition C-B, and theconduction of the phases, are analogous to those in the absorber, i.e.suitable equipment is that in which a gas-liquid phase interface isgenerated, through which heat and mass transfer between the phases isenabled, and which are provided if required with internal or externalequipment for heat supply and/or heat removal.

According to the invention, the desorption can be performed in one ormore stages.

Possible embodiments of the desorber are a simple (flash) vessel andcolumns.

A preferred embodiment of the present invention in which the absorption,i.e. the contacting with the solvent mixture, and the desorption arecombined in one apparatus is, for example, the dividing wall column. Inthis column, the contacting, and the associated absorption, and thedesorption are conducted in countercurrent in a plurality of stages byvarying the temperature, combined with stripping with solvent vapor.Both in (i) and (ii) and in (iii) and (iv), the absorption anddesorption apparatus can be combined, especially in a dividing wallcolumn.

In a preferred embodiment, the present invention therefore relates to aprocess as described above, wherein steps (i) and (ii) or steps (iii)and (iv) or steps (i) and (ii) and steps (iii) and (iv) are performed ina dividing wall column.

In a particularly preferred embodiment of the invention, in step (i),the gas mixture G-0 comprising N₂O is first contacted under elevatedpressure p_(abso) with the solvent mixture S-I in an absorption columnoperated in countercurrent and with random packing, which can result inabsorption, and a composition C-A is obtained. In step (ii), thecomposition C-A, in this embodiment, is transferred into a vessel inwhich the composition C-A is decompressed to a lower pressurep_(deso)<p_(abso). The process is preferably conducted virtuallyisothermally with a temperature difference between absorption anddesorption temperature of not more than 20 K, preferably not more than15 K, especially not more than 10 K. The absorption pressure here isfrom 1 to 100 bar, preferably from 5 to 65 bar, especially from 10 to 40bar, preferably from 10 to 35 bar, more preferably from 13 to 30 bar,even more preferably from about 14 to 25 bar, and the desorptionpressure from 0.1 to 2 bar absolute, preferably from 0.5 to 1.5 barabsolute, more preferably from 1.0 to 1.2 bar absolute.

Preference is likewise given, in step (iii), to first contacting the gasmixture G-1 under elevated pressure p_(abso) with a solvent mixture S-IIin an absorption column operated in countercurrent and with randompacking to obtain the composition C-B. In step (iv), composition C-B istransferred to a vessel in which the composition C-B is decompressed toa lower pressure p_(deso)<p_(abso). The process is preferably likewiseconducted virtually isothermally with a temperature difference betweenthe absorption and desorption temperatures of not more than 20 K,preferably not more than 15 K, especially not more than 10 K. Theabsorption pressure here is from 1 to 100 bar, preferably from 5 to 65bar, especially from 10 to 40 bar, preferably from 10 to 35 bar, morepreferably from 13 to 30 bar, even more preferably from about 14 to 25bar, and the desorption pressure from 0.1 to 2 bar absolute, preferablyfrom 0.5 to 1.5 bar absolute, more preferably from 1.0 to 1.2 barabsolute.

In addition to steps (i), (ii), (iii) and (iv), step (A) of the processaccording to the invention may also comprise further steps. For example,the process may also comprise a further treatment of the gas mixture G-1between steps (ii) and (iii). Such treatments comprise, for example, achange in the temperature or a change in the pressure or a change in thetemperature and in the pressure.

For example, the composition of a gas mixture may change, for examplethrough condensation of one of the components. These components may, forexample, be water or another compound present in the solvent mixtureS-I, preferably a solvent which is used for step (i) in the solventmixture S-I in the process according to the invention.

According to the invention, it is possible that further components areremoved from the gas mixture G-1 or G-2. For example, it is possiblethat traces of water, which may be present in the gas mixture G-2 instep (iv) after the desorption, are removed from the gas mixture G-2 bycompression and subsequent cooling.

In this case, the gas mixture G-2 is advantageously compressed to apressure of from 1 to 35 bar, preferably from 2 to 30 bar, morepreferably from 3 to 27 bar. Cooling is preferably effectedsubsequently, preferably to from 1 to 25° C., more preferably from 3 to20° C., especially from 4 to 15° C., more preferably from 8 to 12° C.

In an advantageous embodiment of the process according to the invention,it is also possible that gas mixtures or solvent mixtures are recycledinto the process according to the invention, in order to reduce yieldlosses.

According to the invention, it is possible, for example, that the gasmixture M-2 is recycled into a stage of the process. In such anembodiment, traces of dinitrogen monoxide which are present in gasmixture M-2 can be recycled into the process in order to avoid yieldlosses.

In a further embodiment, the present invention therefore also relates toa process as described above for purifying a gas mixture comprisingdinitrogen monoxide, wherein the gas mixture M-2 is recycled into step(A).

As described above, the gas mixture M-2 is preferably recycled into step(A) of the process according to the invention. In this case, in thecontext of the present invention, the gas mixture M-2 is mixed withanother gas mixture. Preference is given to recycling the gas mixtureM-2 into step (A) in such a way that recovery of the dinitrogen monoxidewhich may be present in gas mixture M-2 is possible. It is thereforepreferred in the context of the present invention that the gas mixtureM-2 is mixed with a gas mixture which is set to an absorption,especially with the gas mixture G-0 or gas mixture G-1. It is thuspreferred in the context of the present invention to recycle gas mixtureM-2 into step (i) or into step (iii) of step (A).

In a further embodiment, the present invention therefore also relates toa process as described above for purifying a gas mixture comprisingdinitrogen monoxide, wherein the gas mixture M-2 is recycled into step(i) or into step (iii) of step (A).

The pressure in the individual steps of the process according to theinvention is preferably selected such that no pump or compressor isrequired in order to recycle the gas mixture M-2 into step (A).Accordingly, it is preferred that step (II) is performed at a pressurewhich is, for example, from 0.2 to 5 bar higher than the pressure instep (i) or in step (iii).

In the process according to the invention, the proportion of oxygen inthe composition obtained can be reduced significantly. Moreparticularly, in the preferred embodiment comprising the recycling ofthe gas mixture M-2, this is possible in accordance with the inventionwithout reducing the yield of dinitrogen monoxide.

The liquid composition C-2 which comprises dinitrogen monoxide and isobtained by the process according to the invention can in principle beused for all applications in which pure dinitrogen monoxide streams ordinitrogen monoxide streams admixed with inert gas are typically used.More particularly, the composition C-2 is suitable, for example, for theoxidation of methanol to formaldehyde, as described, for example, inEP-A 0 624 565 or DE-A 196 05 211. The present invention therefore alsorelates to the use of the liquid composition C-2 which comprisesdinitrogen monoxide and is obtainable by a process according to theinvention as an oxidizing agent for methanol.

The process according to the invention affords liquid compositionscomprising dinitrogen monoxide which have a particularly low proportionof disruptive secondary components. This is especially advantageous foruse as an oxidizing agent, since, as a result of the low proportion ofdisruptive secondary components, barely any side reactions occur andthus particularly pure products can be obtained in an oxidation. Liquidcomposition C-2 preferably comprises, after the inventive purification,not only dinitrogen monoxide but also carbon dioxide in suitableamounts.

The liquid composition C-2 purified in accordance with the inventioncomprises preferably from 50 to 99.0% by volume of dinitrogen monoxide,from 1 to 20% by volume of carbon dioxide and from 0 to 25% by volume offurther gases. The percentages by volume specified are based in eachcase on the overall composition C-2. The sum of the individualcomponents of the composition C-2 adds up to 100% by volume.

The composition C-2 purified in accordance with the invention preferablycomprises from 60 to 95% by volume of dinitrogen monoxide, especiallyfrom 70 to 90% by volume and more preferably from 75 to 89% by volume ofdinitrogen monoxide.

The composition C-2 purified in accordance with the invention furthercomprises from 1 to 20% by volume of carbon dioxide. The composition C-2preferably comprises from 5 to 15% by volume of carbon dioxide,especially from 6 to 14% by volume of carbon dioxide.

The composition C-2 preferably comprises from 2 to 25% by volume offurther gases and more preferably from 0 to 5% by volume. Thecomposition C-2 purified in accordance with the invention may compriseone or more further gases, the amount specified being based on the sumof the gases present. The composition C-2 may comprise, for example,traces of oxygen, nitrogen and water.

It has been found that, in the presence of CO₂ as an inert gas inliquefied gas mixtures comprising N₂O, compared to other inert gases,significantly smaller amounts of the inert gas, i.e. carbon dioxide, arerequired to ensure safe operation, for example to preventself-decomposition of dinitrogen monoxide.

The present invention therefore also relates to the use of a liquidcomposition C-2 obtainable by a process according to the invention asdescribed above as an oxidizing agent, especially as an oxidizing agentfor olefins.

Suitable olefins are, for example, open-chain or cyclic olefins havingone or more double bonds. Preference is further given to cyclic olefinshaving one or more double bonds, for example cyclopentene, cyclohexene,cycloheptene, cyclooctene, cyclodecene, cycloundecene, cyclododecene,1,4-cyclohexadiene, 1,6-cyclodecadiene, 1,6,11-cyclopentadecatriene,1,5,9,13-cyclohexadecatetraene or 1,5,9-cyclododecatriene.

In a preferred embodiment, the present invention therefore also relatesto use as described above as an oxidizing agent for olefins, wherein theolefin is selected from the group consisting of cyclopentene,cyclododecene and 1,5,9-cyclododecatriene.

The enriched and purified N₂O-containing liquid composition C-2 is veryparticularly suitable for oxidizing olefins to ketones. For thispurpose, the liquid composition C-2 can preferably be reacted directlywith the olefin.

For such applications, it is advantageous when the proportion of insertgases in the liquid composition C-2 is at a minimum, since the reactorvolume is otherwise unnecessarily enlarged.

For the inventive use as an oxidizing agent, especially for olefins, theoxidation can generally be effected by all process regimes in which theoxidation, especially of the olefin, takes place. More particularly,both continuous process regimes and methods of reaction and batchreactions are possible. According to the invention, the reactionconditions for the oxidation are selected such that a reaction takesplace. Pressure and temperature can be selected accordingly.

The pressure is preferably within a range up to 500 bar, for examplefrom 1 to 320 bar, preferably from 10 to 300 bar, especially from 90 to280 bar. The temperature is preferably within a range from 180 to 320°C., for example from 200 to 300° C., especially from 240 to 290° C.

The oxidation can be performed in the presence of a suitable solvent.According to the invention, however, it is equally possible to performthe oxidation without the addition of a solvent.

According to the invention, preference is given to conducting theoxidation, through suitable selection of the pressure and of thetemperature, such that no gas phase occurs in the reaction zone.

The invention will be illustrated in detail hereinafter with referenceto examples.

EXAMPLES Example 1

10 kg/h of liquid dinitrogen monoxide at −13° C., in which 1320 ppm byweight of oxygen were present in dissolved form, was fed at atemperature of −13° C. and a pressure of 26 bar into the top of awell-insulated pilot column of internal diameter 30 mm, which wasequipped with an approx. 500 mm-high bed of random packing consisting ofRaschig rings of diameter 0.5 inch. The column was operated in tricklemode, which means that liquid arriving at the bottom of the column wasdischarged under level control, and the liquid level was below the bedof random packing. Below the bed of random packing, gaseous nitrogen wasintroduced into the column in order to remove the oxygen dissolved inthe liquid dinitrogen monoxide by stripping in countercurrent. TheO₂-laden stripping gas was removed at the top of the column underpressure control.

In the course of operation, the amount of nitrogen was varied and thetotal amount of liquid dinitrogen monoxide leaving at the bottom wasmeasured for each setting. A sample thereof was also taken in each caseto determine the oxygen content.

The following results were achieved:

O₂ dissolved Spec. in the consumption of Bottom bottom N₂ used/O₂Nitrogen feed product product removed Example l/h kg/h ppm by wt.mol/mol 1 2 9.40 550 5.2 2 4 9.11 256 10.5 3 6 8.87 102 15.7 4 8 8.83 7320.9 5 10 8.87 68 26.1

The examples show that, even with small amounts of stripping gas, it waspossible to considerably lower the O₂ content in the liquid dinitrogenmonoxide.

The losses of dinitrogen monoxide with the stripping gas are low and canbe lowered even further by recycling the stripping gas into an earlierprocess stage.

Example 2 Process for Isolating and Purifying N₂O

The source used for the N₂O is the offgas of a nitric acid plant, whichis in turn operated with the offgas of an adipic acid plant andpartially with pure NO. 26.2 t/h of this offgas are first compressed to25 bar and cooled to 35° C. The water which condenses and also comprisessmall amounts of nitric acid is removed and disposed of.

The remaining compressed gas stream (26.1 t/h) comprises 86.4% by volumeof N₂, 8.1% by volume of N₂O, 3.1% by volume of O₂ and 1.1% by volume ofCO₂ as main components. This stream is fed in at the bottom of anabsorption column of height 22.7 m and diameter of 5.5 m, which isfilled with Pall rings. In countercurrent thereto, 2290 t/h of water arefed in from the top at a temperature of 35° C. The unabsorbed gas isdecompressed through a decompression turbine back into the offgas lineof the nitric acid plant.

The laden absorbent is decompressed to 1.1 bar by means of adecompression turbine in the first desorber tower. The desorber towerhas a diameter of 3.6 m and height of 11.1 m and is filled with Pallrings. The water is conveyed back into the absorber tower. In thiscircuit, the pH is kept between 6 and 7 (measured online with calibratedglass electrodes) by adding 25% sodium hydroxide solution. An average ofapprox. 44 kg/h of sodium hydroxide solution are used.

In order to prevent the accumulation of salts (sodium nitrite, sodiumnitrate and sodium hydrogencarbonate), 2 t/h are purged from the watercircuit and replaced with fresh demineralized water. A heat exchanger inthe water circuit is used to regulate the water temperature.

The gas (2.45 t/h) obtained at the top of the first desorber towercomprises 59.5% by volume of N₂O, 24.2% by volume of N₂, 7.5% by volumeof CO₂, 5.2% by volume of H₂O and 3.0% by volume of O₂ as maincomponents. This gas is in turn compressed to 25 bar and cooled to 35°C. The water which condenses is removed and disposed of. The compressedgas stream is then introduced into a second absorber at the bottomtogether with the recycled gas streams from the partial condensation andthe stripping. This absorber has a diameter of 1.9 m and a height of14.3 m and is filled with Pall rings. In countercurrent thereto, water(274 t/a at a temperature of 35° C.) is introduced to the absorber as anabsorbent.

The unabsorbed gas is decompressed and decompressed together with theoffgas of the first absorber in the offgas line of the nitric acidplant.

The laden absorbent is then decompressed to 1.1 bar in the seconddesorber tower. The water is conveyed back into the absorber tower. Inorder to prevent the pH from falling, 225 kg/h are purged from the watercircuit and replaced with fresh demineralized water. A heat exchanger inthe water circuit is used to regulate the water temperature of thewater.

The gas (2.9 t/h) obtained at the top of the second desorber towercomprises 81.7% by volume of N₂O, 10.7% by volume of CO₂, 5.3% by volumeof H₂O, 1.7% by volume of N₂ and 0.45% by volume of O₂ as maincomponents. This gas is in turn compressed to 26 bar and cooled to 13°C. The water which condenses is removed and disposed of.

The compressed gas stream (2.8 t/h) is then passed through an uprighttube bundle heat exchanger which is operated on the jacket side with acooled water/glycol mixture, where it is cooled to −12° C. Thiscondenses a stream (2060 kg/h) which comprises 87.9% by volume of N₂O,11.4% by volume of CO₂, 0.3% by volume of H₂O, 0.3% by volume of N₂ and0.14% by volume of O₂ as main components.

In order to thaw the tubes of the heat exchanger, two parallel heatexchangers are used, which are operated in NB mode. In order toaccelerate the thawing operation, the heat exchangers are provided withan electrical heater. The uncondensed fraction (790 kg/h) comprises81.5% by volume of N₂O, 11.2% by volume of CO₂, 5.6% by volume of N₂ and1.3% by volume of O₂ as main components and is recycled to the inlet ofthe second absorber as already mentioned above.

The condensed stream is then stripped in countercurrent with nitrogen (4kg/h, corresponding to 19 mol of N₂/mol of O₂ in the liquid N₂O feed) ina stripping column which is operated at 26 bar in trickle mode. Thestripping column has a diameter of 0.35 m and a height of 4.15 m and isprovided with a structured metal packing (packing length: 3 m) with aspecific surface area of 350 m²/m³. The stripping gas at the top of thecolumn (260 kg/h) comprises 78.4% by volume of N₂O, 10.8% by volume ofCO₂, 9.6% by volume of N₂ and 1.0% by volume of O₂ as main componentsand is recycled to the inlet of the second absorber as already mentionedabove.

The liquid product at the bottom of the stripping column (1835 kg/h)comprises 86.7% by volume of N₂O, 11.1% by volume of CO₂ and 1.9% byvolume of N₂ as main components and only 100 ppm by volume of O₂.

The use of the stripping column allows the O₂ content in the liquid N₂Oto be reduced by a factor of 14. The molar N₂O to O₂ ratio rises from630 to almost 7300 mol/mol. As a result of the recycling of thestripping gas into the second absorption column, the isolated yieldnevertheless remains high. The isolated yield of N₂O (based on thecompressed gas after the desorption) is 96.2%.

The concentrated and purified N₂O can be used, for example, for theoxidation of olefins, for example of 1,5,9-cyclododecadiene.

Example 3 Influence of O₂ on the Decomposition of1,5,9-cyclododecatriene

In order to study the influence of O₂ on the decomposition of1,5,9-cyclododecatriene, 500 g of technical-grade1,5,9-cyclododecatriene were initially charged in a 1000 ml glass flaskequipped with a magnetic stirrer, a gas inlet tube and a refluxcondenser. The flask was then heated to 180° C. in an oil bath, and 2 l(STP)/h of synthetic air were introduced through the gas inlet tube witha Brooks mass flow meter.

The offgas rate and composition thereof were determined at the outlet.In addition, samples of the liquid were taken and analyzed by gaschromatography at regular intervals. From the offgas analysis, an O₂consumption of 11 mmol/h is calculated. The 1,5,9-cyclododecatrienecontent in the solution decreases at 2%/h. This means that 1.1 mol of1,5,9-cyclododecatriene are destroyed per mole of O₂. Apart from smallamounts of the monoepoxide of 1,5,9-cyclododecatriene, this does notform any defined products but merely polymeric deposits. A control testshowed that, when only nitrogen instead of synthetic air is bubbled in,no decrease in the content of 1,5,9-cyclododecatriene is observed.

This test shows that, even at temperatures significantly below thetemperature which is needed to oxidize 1,5,9-cyclododecatriene with N₂O(approx. 250° C.), oxygen reacts with 1,5,9-cyclododecatriene. Thisforms polymeric deposits which can lead to blockage of the reactor. Itis thus very important to use an N₂O-containing gas mixture whichcomprises a minimum amount of O₂ as an oxidizing agent, in order both toobtain a high selectivity and to prevent deposits in the reactor.

Example 4 Influence of O₂ on the Decomposition of 4,8-cyclododecadienone

In order to study the influence of O₂ on the decomposition of4,8-cyclododecadienone (the product from the oxidation of1,5,9-cyclododecatriene with N₂O), 500 g of 4,8-cyclododecadienone(approx. 98%, as an isomer mixture) were initially charged in a 1000 mlglass flask equipped with a magnetic stirrer, a gas inlet tube and areflux condenser. The flask was then heated to 180° C. in an oil bathand 2 l (STP)/h of synthetic air were passed through the gas inlet tubewith a mass flow meter.

The offgas rate and the composition thereof were determined at theoutlet. In addition, samples of the liquid were taken and analyzed bygas chromatography at regular intervals. From the offgas analysis, an O₂consumption of 8 mmol/h is calculated. The 4,8-cyclododecadienonecontent in the solution decreases at 1.6%/h. This means that 1.2 mol of4,8-cyclododecadienone are destroyed per mole of O₂. This does not formany defined products, but no polymeric deposits either. A control testshowed that when only nitrogen is bubbled in instead of synthetic air,no decrease in the 4,8-cyclododecadienone content is observed.

This test shows that, even at temperatures which are significantly belowthe temperature which is needed to oxidize 1,5,9-cyclododecatriene withN₂O to 4,8-cyclododecadienone (approx. 250° C.), oxygen reacts with4,8-cyclododecadienone (though, as expected, somewhat more slowly thanwith 1,5,9-cyclododecatriene). It is thus important to use anN₂O-containing gas mixture which comprises a minimum amount of O₂ as anoxidizing agent, in order to obtain a high selectivity.

Example 5 Oxidation of 1,5,9-cyclododecatriene with an N₂O-ContainingGas Mixture which Comprises only 200 ppm of O₂

For the continuous oxidation of 1,5,9-cyclododecatriene with N₂O, ajacketed tubular reactor which consists of 7 jacketed tube coilsconnected in series was used. The reaction tube has an internal diameterof 6 mm and each tube coil a length of 5.32 m. The total reaction volumewas accordingly 1.05 liters. Within the jacket is circulated a heatcarrier oil whose temperature is kept constant at 253° C. by means of athermostat. The circulation rate of the heat carrier oil is selectedsuch that the temperature difference between oil input and oil output isless than 2 K. The heat carrier oil is conducted in cocurrent to thereactants. The reactor is provided at the outlet with apressure-regulating valve which keeps the reaction pressure constant at100 bar.

The reactants (1,5,9-cyclododecatriene, commercial product from Degussa,and medical-grade N₂O from Linde, comprises 200 ppm of O₂ according toanalysis) are conveyed by means of suitable metering pumps (membranepiston pumps) and, upstream of the reactor, mixed in a static mixer atroom temperature before they reach the reactor. The feed rates wereadjusted such that the molar ratio between 1,5,9-cyclododecatriene andN₂O at the reactor inlet is 6.2 mol/mol, and the residence time (definedas the volume flow of the reactants at room temperature and 100 bardivided by the reactor volume) is 0.65 hour. The reaction was carriedout until the reactor was at a steady state (approx. 6 hours), beforethe mass balance was commenced. In order to minimize the errors, themass balance time was always 24 hours.

Downstream of the pressure regulation valve, the reactor output wasdecompressed in a cooled (about 20° C.) phase separator, and theproducts (both gas and liquid) were analyzed. The1,5,9-cyclododecatriene conversion was 13.4%. The selectivity for4,8-cyclododecadienone based on 1,5,9-cyclododecatriene was 93.4%.

Example 6 Oxidation of 1,5,9-cyclododecatriene with an N₂O-ContainingGas Mixture which Comprises 400 ppm of O₂ and 8.3% by Volume of CO₂

Example 5 was repeated using, as the reactant, a gas mixture from Lindewhich comprised 8.3% by volume of CO₂ and 400 ppm of O₂ in N₂O.

The 1,5,9-cyclododecatriene conversion was 13.4%. The selectivity for4,8-cyclododecadienone based on 1,5,9-cyclododecatriene was 93.8%.

Within the measurement accuracy, the presence of CO₂ and the slightlyincreased amount thus do not have any significant effect on thereaction.

Comparative Example 7 Oxidation of 1,5,9-cyclododecatriene with anN₂O-Containing Gas Mixture which Comprises 2% by Volume of O₂

Example 5 was repeated using, as the reactant, a mixture from Lindewhich comprised 2% by volume of O₂ in N₂O.

It was impossible to conduct the reaction stably with this feed. Thepressure difference over the reactor rose continuously, and the test hadto be stopped after 72 hours because the reactor was blocked. The firstcoil was then deinstalled and sawn into sections. It was found that thetube was almost completely blocked with polymeric deposits between 30and 80 cm downstream of the reactor inlet.

With such high concentrations of O₂ in N₂O, stable operation of the1,5,9-cyclododecatriene oxidation is impossible.

Comparative Example 8 Oxidation of 1,5,9-cyclododecatriene with anN₂O-Containing Gas Mixture which Comprises 1300 ppm by Volume of O₂

Example 5 was repeated using, as the reactant, a gas mixture from Lindewhich comprised 1300 ppm by volume of O₂ in N₂O.

The reaction was operable stably with this feed over 426 h. The pressuredifference over the reactor remained constant. For inspection, the firstcoil was in turn deinstalled and sawn into sections. No polymer haddeposited on the walls.

The 1,5,9-cyclododecatriene conversion at 14.6% was higher than in theinventive experiment (13.4%), though the selectivity for4,8-cyclododecadienone was only 91.5%, i.e. almost 2% lower than in theinventive experiment (93.4%).

This experiment shows yet again the importance of minimizing the O₂content in the N₂O used.

The invention claimed is:
 1. A process for purifying a gas mixture comprising dinitrogen monoxide, at least comprising the steps of: (I) at least partially condensing a gas mixture G-I comprising dinitrogen monoxide to obtain a liquid composition C-1 comprising dinitrogen monoxide, and (II) contacting the composition C-1 with a gas mixture M-1 to obtain a composition C-2 and a gas mixture M-2, wherein the gas mixture M-1 is selected from the group consisting of nitrogen, helium, neon, argon, krypton, xenon, hydrogen, carbon monoxide, methane, tetrafluoromethane, and mixtures thereof.
 2. The process according to claim 1, wherein step (II) is performed continuously.
 3. The process according to claim 1, wherein step (II) is performed in a bubble column.
 4. The process according to claim 3, wherein the bubble column is operated in countercurrent.
 5. The process according to claim 1, wherein the gas mixture G-I is obtained by a process comprising the steps of: (A) treating a gas mixture G-0 comprising dinitrogen monoxide to obtain a gas mixture G-A, at least comprising the steps of (i) absorbing the gas mixture G-0 in a solvent mixture S-I to obtain an offgas stream and a composition C-A and (ii) desorbing a gas mixture G-1 from the composition C-A to obtain a solvent mixture S-I′.
 6. The process according to claim 5, wherein step (A) additionally comprises steps (iii) and (iv): (iii) absorbing the gas mixture G-1 in a solvent mixture S-II to obtain an offgas stream and a composition C-B (iv) desorbing a gas mixture G-2 from the composition C-B to obtain a solvent mixture S-II′.
 7. The process according to claim 5, wherein the gas mixture M-2 is recycled into step (A).
 8. The process according to claim 6, wherein the gas mixture M-2 is recycled into step (i) or into step (iii) of step (A). 